Parsec - definition of parsec by The Free Dictionary Parsec - definition of parsec by The Free Dictionary (pär′sĕk′) n. A unit of astronomical length based on the distance from Earth at which stellar parallax is one second of arc and equal to 3.258 light-years, 3.086 × 1013 kilometers, or 1.918 × 1013 miles. (ˈpɑːˌsɛk) n (Units) a unit of astronomical distance equal to the distance from earth at which stellar parallax would be 1 second of arc; equivalent to 3.0857 × 1016 m or 3.262 light years [C20: from parallax + second2] (ˈpɑrˌsɛk) n. a unit of distance equal to 206,265 times the distance from the earth to the sun, or 3.26 light years.

[1910–15; par (allax) + sec (ond)2] par·sec (pär′sĕk′) A unit of astronomical length equal to 3.26 light-years. It is based on the distance from Earth at which a star would have a parallax of one second of arc. parsec 1. (pc) The distance at which 1 au would measure 1 sec of arc, equal to 19.16 trillion mi (30.857 trillion km) or 206,265 au or 3.26 ly (1). 2. (pc) A unit of length used for measuring astronomical distances. 1 parsec = 3.26 light years. ThesaurusAntonymsRelated WordsSynonymsLegend: Cool Cosmos Distance Measure In astronomy, the most commonly used measures of distance are the light year, parsec and astronomical unit. Astronomical Unit (A.U.) An astronomical unit (A.U.) is the average distance between the Earth and the Sun, which is about 93 million miles or 150 million kilometers. Astronomical units are usually used to measure distances within our solar system.

For example, the planet Mercury is about 1/3 of an A.U. from the Sun, while the farthest planet, Pluto, is about 40 A.U. from the Sun (thats 40 times as far away from the Sun as the Earth is). Light-Year (ly) Most objects in space are so far away, that using a relatively small unit of distance, such as an astronomical unit, is not practical. Instead, astronomers measure distances to objects which are outside our solar system in light-years. A light-year (ly) is the distance that light can travel in one year in a vacuum (empty space). The speed of light is about 186,000 miles or 300,000 kilometers per second. So, in one year light travels a distance of about 5,880,000,000,000 miles or 9,460,000,000,000 kilometers or 63,240 A.U.. This distance is 1 light-year. For example, the nearest star to us is about 4.3 light-years away.

Our galaxy, the Milky Way, is about 150,000 light-years across, and the nearest large galaxy, Andromeda, is 2.3 million light-years away. Parsec (pc) Astronomers use another unit of distance called a parsec (pc), which is equal to 3.26 light-years. The definition of the parsec is based on a triangle. A parsec is the length of the long leg of a right triangle, whose short leg is one astronomical unit when the angle between the Sun and the Earth, as seen from an object in space (a star for example), is one arcsecond The word parsec stands for "parallax of one arc second" One kiloparsec (kpc), is one thousand parsecs. One megaparsec (Mpc) is one million parsecs. SUMMARY Properties of Stars Distances---Trigonometric Parallax Unlike the other sciences, astronomy is entirely observational. You cannot run experiments on things. You cannot manipulate the objects to see how they work. In order to compare the objects you see from the Earth, you must first know how far away they are.

Obviously, you cannot use a tape measure or send spacecraft out to the stars and measure how far they travelled. Bouncing radar off the surfaces of stars would not work because: (1) stars are glowing balls of hot gas and have no solid surface to reflect the radar beam back and (2) the radar signal would take years to just reach the nearest stars. A favorite way to measure great distances is a technique used for thousands of years: look at something from two different vantage points and determine its distance using trigonometry. The object appears to shift positions compared to the far off background when you look at it from two different vantage points. The angular shift, called the parallax, is one angle of a triangle and the distance between the two vantage points is one side of the triangle. Basic trigonometric relations between the lengths of the sides of a triangle and its angles are used to calculate the lengths of all of the sides of the triangle. This method is called trigonometric parallax. Modern surveyors use this method to measure great distances, so the method is sometimes called ``the surveyor's method''. The side of the triangle between the observers, labelled ``B'' in the figure above, is called the baseline.

The size of the parallax angle p is proportional to the size of the baseline. If the parallax angle is too small to measure because the object is so far away, then the surveyors have to increase their distance from each other. Ordinarily, you would have to use trigonometric functions like a tangent or a sine, but if the angle is small enough, you find a very simple relation between the parallax angle p, baseline B, and the distance d: p = (206,265 × B)/d, where the angle p is measured in the tiny angle unit called an arc second. The farther away the object is, the less it appears to shift. Since the shifts of the stars are so small, arc seconds are used as the unit of the parallax angle. There are 3,600 arc seconds in just one degree. The ball in the tip of a ballpoint pen viewed from across the length of a football field is about 1 arc second. Trigonometric parallax is used to measure the distances of the nearby stars. The stars are so far away that observing a star from opposite sides of the Earth would produce a parallax angle much, much too small to detect.

As large a baseline as possible must be used. The largest one that can be easily used is the orbit of the Earth. In this case the baseline is the distance between the Earth and the Sun---an astronomical unit (AU) or 149.6 million kilometers! A picture of a nearby star is taken against the background of stars from opposite sides of the Earth's orbit (six months apart). The parallax angle p is one-half of the total angular shift. However, even with this large baseline, the distances to the stars in units of astronomical units are huge, so a more convenient unit of distance called a parsec is used (abbreviated with ``pc''). A parsec is the distance of a star that has a parallax of one arc second using a baseline of 1 astronomical unit. Therefore, one parsec = 206,265 astronomical units. The nearest star is about 1.3 parsecs from the solar system. In order to convert parsecs into standard units like kilometers or meters, you must know the numerical value for the astronomical unit---it sets the scale for the rest of the universe. Its value was not know accurately until the early 20th century (see the planetary science chapter ).

In terms of light years, one parsec = 3.26 light years. Which unit should you use to specify distances: a light year or a parsec? Both are fine and are used by astronomers (including myself) all of the time, just as you may use ``feet'' and ``yards'' in everyday talk. However, when the parsec unit is used on this web site, the equivalent in light years will usually be given as well, but when the light year unit is used, the parsec equivalent will usually not be given. Using a parsec for the distance unit and an arc second for the angle, our simple angle formula above becomes extremely simple for measurements from Earth: p = 1/d Parallax angles as small as 1/50 arc second can be measured from the surface of the Earth. This means distances from the ground can be determined for stars that are up to 50 parsecs away. If a star is further away than that, its parallax angle p is too small to measure and you have to use more indirect methods to determine its distance. Stars are about a parsec apart from each other on average, so the method of trigonometric parallax works for just a few thousand nearby stars.

The Hipparcos mission greatly extended the database of trigonometric parallax distances by getting above the blurring effect of the atmosphere. It measured the parallaxes of 118,000 stars to an astonishing precision of 1/1000 arc second (about 20 times better than from the ground)! It measured the parallaxes of 1 million other stars to a precision of about 1/20 arc seconds. Selecting the Hipparcos link will take you to the Hipparcos homepage and the catalogs. The actual stellar parallax triangles are much longer and skinnier than the ones typically shown in astronomy textbooks. They are so long and skinny that you do not need to worry about which distance you actually determine: the distance between the Sun and the star or the distance between the Earth and the star. Taking a look at the skinny star parallax triangle above and realizing that the triangle should be over 4,500 times longer (!), you can see that it does not make any significant difference which distance you want to talk about. If Pluto's entire orbit was fit within a quarter (2.4 centimeters across), the nearest star would be 80 meters away!

But if you are stubborn, consider these figures for the planet-Sun-star star parallax triangle setup above (where the planet-star side is the hypotenuse of the triangle): the Sun--nearest star distance = Units in Astronomy How Distances are Measured in Astronomy Dealing with the numbers involved with the distances to the stars or even with those found in the solar system can be hard going. Astronomers make their lives easier by using a number of rulers (units of distance) for the distances and although they have some strange names they can be very useful for comparing the distances to stars, other galaxies and even the planets in our solar system. AU (astronomical units) One AU is the distance that the average distance that the Earth orbits the Sun at. The AUis most commonly used for the distances of objects with in our solar system. Pluto, the last planet in the solar system is found at an average distance of 39.47 au from the Sun. Sedna the new body nearly as large as a planet found beyond Pluto is never nearer to the Sun than 76AU and then goes to 880AU from the Sun in its giant elliptical orbit.

Light Years One of the most common rulers is the light year. The light year is is the distance that light travel in one year (365 1/4 days). It is most commonly used for the distances to stars and other galaxies. The nearest star is 4.2 light years away from our sun. We are 8.3 light minutes away form the Sun. the distance to the outer most planet Pluto is about 13 light hours. Some other interesting distances in light years: Object STAR WARS: Standard Units Standard Units This page is intended to be a brief summary of our present knowledge about the units of measurement employed in the STAR WARS galaxy. Some of the following information is derived directly from the films; this is irrefutable canon. Some of it depends on planetological and other data printed in references from STAR WARS: The Roleplaying Game . The original invention of those figures was probably arbitrary, and the consequent results presented here are only as solid as the reader's willingness to accept the RPG material. Thanks are due to, in alphabetic order: Dave Baughman for suggesting MGLT as a measure of uncompensated acceleration.

Robert Brown for the details of the SW Monopoly coins, for various general discussions, for proposing the calaculation of the realistic MGLT, and for the suggestion of the image-link icon. Stewart Brown for information about the use of "megalight" in X-Wing: Alliance Scott Griffith for requesting refinement of the explanation of weight. Daniel Haughton contributed the linear [acceleration] tolerance acronymn to rationalise MGLT. Makis Kalofolias for extensive and exhaustive analysis of the units used in the LucasArts Eric Meadows for reminding me about pricing of some items in novels etc. Francisco Cafferata for speculation on the meaning of the computer game unit acronyms. Leonid Panfil for providing in-game statstics about missile weapons in TIE Fighter Wayne Poe for reminders about some of the distance scales stated in A New Hope . Aden Steinke for advice on economics, most of which will require additional web pages if it's to be treated properly. Charbel Tengroth for empirical determination of the MGLT. Michael Wong for the "speed" chart from Behind the Magic . The units of distance measurement employed in the STAR WARS galaxy are metric, based on the metre.

In the A New Hope novel, for instance, Darth Vader is described as being over two metres tall, lightsabre beams are a metre long, and womp-rats are two or three metres in size. Starfighter pilots and their targeting computers use metric metres. [Eg. Luke Skywalker's reference to womp rats in A New Hope ; General Dodonna's reference to a thermal exhaust port.] The number of digits on the targeting displays for rebel X-Wing proton torpedos indicate that these weapons have an effective range of up to a million metres; a thousand kilometres. The reason why pilots at the Battle of Yavin were unable to fire until almost zero-range is because they flew near the station's surface. Flying near the surface put the target exhaust port over the horizon. Approaching the port from above the station's surface would have denied the pilots cover from the surrounding turbolaser batteries. With anything other than the trench-strafing approach, they would have been shot down before gaining a target lock. Different units are used in flight simulation computer games like X-Wing and TIE Fighter . Although the dimensions of starships in the games are stated in terms of metres or kilometres, an unstated new unit is used to measure separations between vessels.

However, as noted in the propulsion section below, the dynamics of these games are not quite the same as in the "real" STAR WARS universe, and the units have been contrived to maintain playability. For special purposes, such as interstellar travel, there are supplementary units in use. One is the parsec, which is an astronomical unit characterising the distances between stars. A parsec is defined to be the distance at which the average separation between Earth and Sun would subtend an angle of one arcsecond (1/3600 of a degree) on the sky. The corresponding unit in the Galactic Empire would be based on the orbit of Coruscant rather than Earth. We don't yet have any way of determining what that measurement is; it could just as easily be slightly less or slightly greater, depending on the mass of Coruscant's sun. We provisionally assume that the orbital semimajor axis is the same. A terrestrial parsec equals 3.086x1016m. Han Solo boasted that his freighter, the Millennium Falcon, was capable of accomplishing a feat called "the Kessel Run" in "less than twelve parsecs".

(The 'Run involves travel through or past an unnavigable region; taking a route of shorter distance requires a good ship and skillful pilot.) A light-year is the distance travelled by light in one year. An Earth-based light-year is 9.461x1015m. However this is not appropriate in a STAR WARS context, because the galactic standard year is derived from the orbital period of Coruscant rather than Earth. As stated below in the time section, the standard year is slightly longer than the terrestrial equivalent. We assume that the standard second exactly equals the usual metric second. Therefore when a citizen of the Old Republic, Galactic Empire or New Republic quotes a distance in "light-years", the unit is actually 368/365.25 times our version of the light-year. (It is about 0.75% greater.) By the Coruscant definition, a light-year is approximately 9.531x1015m. In Shield of Lies , Lando Calrissian states that the galaxy is 120,000ly in diameter.

m km ly pc m 1 0.001 1.049x10-16 3.240x10-17/a km 1000 1 1.049x10-13 3.240x10-14/a ly 9.531x1015 9.531x1012 1 0.3088/a pc 3.086x1016a 3.086x1013a 3.238a 1 The value a=(Coruscant orbit semimajor axis)/(Earth orbit semimajor axis) is assumed to equal 1. X-Wing proton torpedo target scope has six digits; according to the scale of the battle station and the length of the trench (roughly one eighth of the global circumference) the units of measurement are almost certainly metres. Furthermore the pilot briefing given by General Jan Dodonna was expressed in terms of metres. Time The galactic standard second appears to be identical to the metric second.

Higher units of time are reckoned according to the clock and calendar of Coruscant. A standard minute consists of 60 seconds; an hour consists of 60 minutes; a day consists of 24 hours. A standard week consists of 5 days; a month consists of seven weeks. A standard year is 368 days, including ten months, three fete weeks and three other holidays. The dating systems of the Galactic Republic and Galactic Empire are unknown. At least two proposals for dating standards have been used in non-canonical literature. Both are unsatisfactory: In Timothy Zahn's novel Dark Force Rising , a calendar based on the Emperor's rise to power was used. Unfortunately there are a number of continuity problems with Zahn's dating. The worst problem is that he has Vader's adult career well underway several years before Episode I Mass Nothing is known with certainty about the units of mass used in Palpatine's galaxy. By default, we should assume that the base unit is the metric kilogram. At first thought, some might wonder whether the kilogram would be redefined because of Coruscant's surface gravity being higher than Earth's. This is just a confusion between the concepts of mass and weight.

Weight is the force on an object due to gravity; mass is the amount of substance in the object. Force is mass multiplied by acceleration (such as the local acceleration due to gravity). Weight is equal to the mass times the strength of the local gravitational field. The metric unit of force is the newton (N), which equals 1kg.m/s². Obviously a different name would be used for the same unit in STAR WARS , since Sir Isaac Newton probably isn't remembered in that galaxy and era. Mechanical scales do not directly measure mass. They measure the weight force. The mass reading is given according to a calibrated scale which assumes that the local gravity has a certain fixed value. The definition of mass itself is quite independent of weight and a planet's gravitational conditions. Money Money in the Old Republic, Galactic Empire and New Republic takes the form of "galactic credits standard". This is a universal unit of currency. (For a few years after Palpatine's fall, though, there were competing New Republic and Imperial currencies.) In A New Hope , credits took the form of coins (as thrown from Han Solo to a bartender).

The STAR WARS Monopoly CD-ROM states that although the galactic credit is standardised, local mints (presumably operating at a planetary or sector level) produce coins bearing regional symbols and labels. Roleplaying sources state that sophisticated electronic devices called credit chips are also used to record credit transactions. Credit chips are supposed to have nearly impregnable security and encryption. Credit notes akin to those used on Earth are not known. Han Solo took his reward from the Yavin rebels in a form contained in heavy boxes of approximately the same dimensions as a shoe box. The total capacity of all these containers looked to be at least a ton. The material within the boxes was probably a commodity with a stable price; ordinary credit coinage or chips surely would not be so voluminous. The radio dramatisation suggests that the material was a precious metal. Solo and the rebels probably chose this form of transaction because actual credits could potentially be traced by Imperial Intelligence analysts. There are very few canonical indicators of the value of the galactic credit. Ten thousand credits is considered to be excessively expensive as a fare from Tatooine (in the Outer Rim) to Alderaan (a Core world).

A modestly priced transgalactic voyage along a fast hyperlane would take a few days and cost only a few thousand credits. A second-hand hyperspace-capable starship costs more (but not substantially more) than ten thousand credits. Luke Skywalker sold his worn-out landspeeder for approximately 2000 credits (the advance fee demanded by Han Solo for passage to Alderaan). Jabba the Hutt was known to pay 35000 credits bounty for live delivery of one of his wookiee debtors. Drinks at a Mos Eisley cantina cost several coins' worth, but the credit value of the individual coins is unknown. There are numerous statements of credit value in official (but non-canon) sources. The roleplaying game has extensive tables of pricing. In novels, the training of a stormtrooper officer costs over half a million credits; ten million credits buys a wing of starfighters; C-3PO objectively values himself at 43000 credits, and his constituent parts as five thousand. A "decicred" coin, which is valued at a tenth of a credit, is flipped for gambling purposes [ Rebel Dawn p.310].

There is zero nett inflation in the galactic economy. The credit retained roughly equal value over at least four millenia. Compare stated prices of goods and services stated in Tales of the Jedi Companion with prices for similar things in the films, unfilmed fiction and roleplaying material of the Palpatine Era. The absence of inflation for over 160 generations indicates extraordinary stability which deserves detailed examination. This fact is a key hint at the nature of the galactic economy. This should be discussed elsewhere. Credits paid by Han Solo to the cantina bartender appeared to be rectangular metallic coins of some kind. A similar transaction took place when Luke Skywalker sold his landspeeder, but unfortunately Luke and the buyer were too far from camera for the exchanged tokens to be discerned. However, the exchange did involve items fitting into the palm of a hand, which were probably coins. Images of various 1000-credit coins from the STAR WARS Monopoly game. The two lowermost golden coins bear the starburst in diamond logo of the Corporate Sector Authority.

Wupiupi coins of Tatooine from The Phantom Menace , during the period when Hutt syndicates dominated the planet and controlled its economy by issuing their own hard currency in opposition to the credits of the Old Republic. Han Solo's reward from the rebels at yavin took the form of a bulky commodity stored in boxes. In the A New Hope radio dramatisation it was suggested that the goods were rare metals. Bounty hunting is an important profession in the lives of disreputable citizens of the galaxy. Gangsters are willing to place generous bounties on their debtors. Even senior advisors of the Emperor are not above hiring mercenaries to capture important subversives and criminals. The hunters are able to pursue their prey across the galaxy and make a living for rewards of only tens of thousands of credits (e.g. Chewbacca's reward in ROTJ Propulsion Both the flight-simulation computer games and the starship rules of the roleplaying game sacrifice realism in propulsion for the sake of the commonsense intuitions of air-dwelling players. The games implicitly assume the dynamics of vehicles flying in a resistive medium. In other words, it appears as if maintaining a velocity requires effort.

In real empty space an object remains in motion at constant speed and direction until it is acted upon by a force; it does not dwindle to a halt when thrust is turned off. A starfighter's drive systems actually cause straight-line acceleration, deceleration, and/or changes of direction and orientation. For realistic technical purposes the games' speed statistics should be regarded as either acceleration, thrust or engine power; the "acceleration" statistics are actually the ability to vary acceleration; and the manoeuvrability statistics should remain as measures some kind of maximum manoeuvring torque. The propulsion statistics are usually stated in terms of arbitrary game-specific units that cannot be readily related to the real world. The computer games use a "speed" unit called the MGLT, which looks like it might be an acronym. The roleplaying game does not even label its distance and speed units. In fact the roleplaying units are said to take different values depending on the difficulty of movement due to local conditions, hazards and obstructions. The statistics are simply meant to convey the idea that some vessels are more nimble or are capable of outrunning other kinds of vessels.

The only exception, when speeds are stated in terms of absolute metres per five seconds or kilometres per hour, is for atmospheric or ground movement. The MGLT was used (and probably invented by) ILM artists long before the development of the modern computer games. The earliest known use of the term is in a diagram comparing the propulsive capabilities of starfighters, drawn during the production of Return of the Jedi . This illustration is reproduced in Return of the Jedi Sketchbook . As an acronym, the unit probably refers to something in the production of the movies' special effects; the "GL" could stand for "George Lucas". As a measurement of the maximum straight-line acceleration of a starship in space, it might (speculatively) be an acronym for something like "Micro-Gravity Linear [acceleration] Tolerance", as suggested by one reader. The instruction booklet for the game X-Wing: Alliance uses the phrase "megalights per hour" which is probably a colloquial second-generation attempt to rationalise the MGLT by inserting vowel sounds, but this might not have been the original ILM intention. Judged on this basis, rather than the unrealistic kinematics of the computer games, the MGLT must be a measurement of acceleration.

Because a star destroyer can keep pace with the Millennium Falcon in a straight-line chase, we know that it must have approximately the same statistic: 75 MGLT, or better if Solo was stressing his freighter beyond its safe limits. Since the Imperial fleet in Return of the Jedi demonstrated a deceleration of not less than 30km/s², [see Star Destroyers ] 1 MGLT must equal at least 400m/s². This is about 40g in terms of Earth gravity, or 29gs in terms of Coruscant gravity. A similar theory is that the MGLT may describe the residual acceleration felt by the crew of a spaceship, reduced from the full inertial force by the action of acceleration compensators (inertial dampers). This would complicate the specifications by convolving the propulsive capabilities with the precision and capacity of inertial dampers. The value of 1MGLT would have to be significantly less than the acceleration due to a standard gravity, because starfighters are known to perform at over 100MGLT and yet acclerations greater than a few standard gravities are harmful to human health.

Comparing the MGLT propulsion ratings from LucasArts and ILM with the roleplaying game statistics is not trivial. By performing regression analysis on the data for several known rebel and Imperial fighters and bombers, it is possible to derive approximate mathematical expressions relating these three different acceleration ratings. The ILM rating in MGLT is proportional to the LucasArts rating raised to the power of 2.4. Thus the TIE Avenger and Defender are 240MGLT and 282MGLT on the ILM scale. The WEG rating is weakly related to the other standards. If the LucasArts MGLT rating is L then the WEG rating is in the range W±1, where W=0.14L-5.5 approximately. Thus the WEG propulsion ratings of the Avenger and Defender cannot be less than 14 and 16 respectively. For a discussion of the measurement of hyperspace capabilities, refer to Hyperspace : Duration and Speed .

m/s² gs MGLT m/s² 1 0.072 <0.0025 gs 13.93 1 <0.035 MGLT >400 >29 1 Original starship performance diagram from ILM, drawn for the production of Return of the Jedi Computer Games It must be stressed again that the use of speed-based statistics rather than realistic accelerative propulsion is simply a convention for the comfort of players. Other statistics in the computer games, including hull, shield and manoeuvrability codes, are also meant to imply that starships have different capabilities but may reflect the realistic STAR WARS universe only as well as the propulsion figures. Nevertheless, it is useful to consider what the game characteristics are, as well as what they realistically ought to be. The magnitude of the MGLT can be determined by flying past large warships of known length at a fixed speed of 100MGLT whilst making careful timing measurements. Charbel Tengroth has performed these experiments and found that 100MGLT = 100m/s, and the relationship appears to be linear in the interval of 50-150MGLT.

So in terms of gameplay, 1MGLT is one metre per second. Makis Kalofolias has tested the game program further to determine that the game-speed MGLT equals 1m/s under all circumstances. He also finds that the internal accelerations statistics, expressed in MGLT/s are the same as the metric acceleration unit 1m/s². His investigations also show that manoeuverability is actually two characteristics: a pitch rate (P) and a roll rate (R). The overall manoeuverability statistic is R+2P, and is measured in a unit called DPF. The meaning of this acronym is unknown, but it might be "degrees per frame" or something similar. Mr Kalofolias also calibrated the units for armoured durability of hulls (RU, possibly "resistance units"), shield base durability (SBD), and the potency of weapons. It turns out that 1RU has the same effectiveness as 1SBD for resisting damage by starfighter weapons. That is the ideal behavior, but the game engine introduces some irregularities in order to provide a subtle sense of variation in the simulated combat.

The unnamed units of weapon damage, in terms of which an ordinary proton torpedo is 10000, are destructively equivalent to 0.01 of a RU or SBD, ie. 1RU = 1SBD = 100 units of weapon destructiveness. If we define a basic unit of destructive potential (DP) equal to 1RU or 1SBD, some useful statistics for comparison are: OBJECT RATING (DP) proton torpedo 100 heavy rocket 350 space bomb 650 ISD shields 4800 ISD hull ~2300 ISD refers to the common Imperator-class star destroyer. In TIE Fighter the player is often allowed the choice of missile payload for missions invoving bombers or deluxe fightercraft. In this interface, blast ratings and propulsive capabilities are listed for each option. These statistics are roughly equivalent to those presented above based on internal inspection of the game engine. WEAPON RATING (BLS) PROPULSION (MGLT) adv. concussion missile 60 280 proton torpedo 120 100 (?) adv.

proton torpedo 150 space rocket 350 heavy space bomb 600 Standard values refer to the planet Coruscant, which is the capital of galactic civilisation and is considered the ideal inhabitable world for humans. Galaxy Guide 2: Yavin & Bespin claims that standard planetary density is 5.63 times that of water; ie. 5630kg.m-3. The book also contains several definite statements of measured statistics of particular planets and moons. Combining these figures with the known universal physical constants allows us to deduce the real values of the rest of the galactic standard units for planetary measurement. The gas-giant planet Yavin has radius R=96239km, mass M=323.7Ms, and surface gravity g=2.74gs. Its year is slightly greater than 13 standard years, and its sidereal day is slightly less than 12 hours. The system is 7.5 billion years old. Combining the statistics for surface gravity and planetary mass yields an equation relating the standard units of surface gravity and planetary mass.

gs / (m.s-2) = Ms / (1.175x1024kg) Taking note of the fact that standard surface gravity, planetary mass and planetary radius are actually the exact properties of a single planet, namely Coruscant, the standard values must equal: gs = 13.93 m.s-2 Ms = 1.637x1025kg Rs = 8854km For comparison, the mass of Earth is 5.974x1024kg, and its average surface gravity is 9.807 m.s-2. Therefore, Coruscant's gravity is 1.421 times that of Earth; its mass is 2.74 times Earth's; and its radius (and diameter) are 1.389 times Earth values. The surface area of Coruscant is almost double the surface area of Earth (1.929 times). On a miscellaneous note: antigrav range for a habitable planet is within approximately six planetary diameters. In other words, within this range repulsorlift is the propulsion method of preference.

For an Earth-sized body, this range is just under eighty-thousand kilometres. For further information about the filmed planets of STAR WARS Astronomical Distances by Ron Kurtus - Succeed in Understanding Astronomy: School for Champions Astronomical Distances by Ron Kurtus (15 March 2009) The distances in Astronomy are so great that using miles or kilometers is insufficient, because the numbers become too large to handle. When dealing with the great distances within our Solar System, astronomical units (AU), which are multiples of the distance from the Earth to the Sun, are used. However, that unit of measurement is not large enough when considering the distance to other stars or galaxies. In that case, distance is stated in light years, which is how far light travels in a year. A third unit that is preferred by astronomers is the parsec, which is even greater than a light year. Questions you may have include: How far is an astronomical unit? How far is a light year? Why is a parsec used? This lesson will answer those questions. Astronomical units The unit of measurement that is convenient for stating the large distances within our Solar System is the astronomical unit (AU).

It is defined as the distance from the Earth to the Sun. That distance is approximately 150 million kilometers (150,000,000 km) or 93 million miles (93,000,000 mi). Some distances within the Solar System in AU are: By definition, the Earth is 1 AU from the Sun Mars is 1.52 AU from the Sun Jupiter is 5.20 AU from the Sun Pluto is about 39.5 AU from the Sun The distance between planets depends on their orientation in their orbits. Mars can be between 2.52 AU and 0.53 AU from Earth, depending on their relative positions. Distances from Sun of Mars and Earth As of 2009, the American spacecraft Voyager 1 had traveled over 108 AU from the Sun, the furthest a man-made object has traveled. The distance of 108 AU equates to 16,200,000,000 km or 10,044,000,000 miles.

Light year Although the astronomical unit is fine for our Solar System, it is not sufficient to designate the greater distances to other stars and galaxies. Instead, the light year is used as a unit of measurement. A light year is the distance light travels in one year and can be calculated as the speed of light in kilometers/second or miles/second multiplied times the number of seconds in a year. The speed of light is approximately 300,000 kilometers per second or 186,000 miles per second. One year equals 365 days × 24 hours in a day × 60 minutes in an hour × 60 seconds in a minute, which equals 31,536,000 seconds. Thus, a light year is about: 9,500,000,000,000 km or 9.5*1012 km 5,900,000,000,000 mi or 5.9 *1012 mi A light year is also equals 63,241 AU.

Common large distances Common large distances in space, measured in light years, include: Proxima Centauri, the nearest star in our Milky Way galaxy, is 4.22 light years away. The Milky Way galaxy is about 100,000 light years across. The Andromeda Galaxy is approximately 2,500,000 light years away. The size of the Universe is estimated to be between 93 billion and 156 billion light years across. The time it takes light to travel from the Sun to the Earth (1 AU) is approximately 499 seconds or 8.32 minutes. You could say that 1 AU equals 8.33 light minutes. Parsec Astronomers prefer to use the parsec to designate extremely great distances in space, because it relates to the geometric method they commonly use to establish distance. Parsec stands for parallax of one arcsecond. It is 1 AU divided by the tangent of an arcsecond. Parallax effect determines angle As an explanation of the geometric method used to measure distance with the parallax effect, consider the situation when you observe a distant object and move side-to-side. The object seems to move or change position.

This effect can be used to determine the resulting angle from moving side-to-side as compared to the distant object. That angle, in turn, is used to calculate the distance. For example, knowing the distance A and the angle a in the illustration below, you can find the distance B using simple trigonometry. Tangent of angle used to determine length of side For distant objects in space, the angle becomes very small. An acrsecond is equal to 1/60 of an angular degree. Astronomers can measure angles as small as 0.001 arcsecond. Parsec is related to AU In defining the parsec, the distance from the Earth to the Sun (1 AU) is used as the base of the right triangle and the arcsecond is used as the angle. This is because measurements are made in two parts of the Earth's orbit around the Sun, in order to get as great a distance for the base as possible. Hypothetical star at 1 parsec from Sun Thus, 1 parsec = 1 AU / tan(1 arcsecond). At some given time of the year, an astronomer can line up a distant star or galaxy with a nearby star of known distance from the Earth.

Then, six months later, when the Earth is on the other side of its orbit around the Sun, the astronomer will line up the distant star with the known star. But because of the parallax effect, the distant star will seem to have moved and will be an a different angle from the Earth. By measuring that angle and making some calculations, the astronomer can determine the distance to the far-off star or galaxy. Parsec not popularly used Although astronomers prefer to use the parsec, other scientists and the general public use the light year to designate large distances. The reason is that it is difficult to visualize a distance in terms of a small angle. Also, one parsec is approximately 3.262 light-years, so there isn't much advantage in using the term. However, those working in Astronomy must be familiar with the unit of measurement. Summary The great distances measured in Astronomy require units much larger than kilometers or miles. For distances within our Solar System, astronomical units (AU) or multiples of the distance from the Earth to the Sun are used. For distances to stars or galaxies, light years are used. A third unit that is preferred by astronomers is the parsec, which is even greater than a light year in distance.

Go the distance How the Star Wars Kessel Run Turns Han Solo Into a Time-Traveler | WIRED How the Star Wars Kessel Run Turns Han Solo Into a Time-Traveler subscribe 6 months for $5 - plus a FREE Portable Phone Charger. On Twitter 8 hours Slack has started rolling out threaded messaging, which allows users to connect related messages in a given chatroom bit.ly/2joimKu Author: Kyle Hill. Kyle Hill Culture Date of Publication: 02.12.13. Time of Publication: 6:30 am. 6:30 am How the Star Wars Kessel Run Turns Han Solo Into a Time-Traveler image You’ll hear any reputable Star Wars fan point it out eventually: Han Solo’s famous boast that the Millennium Falcon “made the Kessel Run in less than 12 parsecs” may have sounded impressive, but from an astronomical perspective, it made no sense. A parsec is a unit of distance, not time, so why would Solo use it to explain how quickly his ship could travel? There are two stories going on here. The first is that Solo’s famous line of dialog was simply a mistake of terminology.

The second — the one I choose believe — is far more interesting, because it means that when Obi-Wan sat down across from the wryly smiling Han Solo in that cramped cantina, he met a time-traveling smuggler born at least 40 years before the events of The Phantom Menace ever took place. A Parsec by Any Other Name First coined in 1913 by British astronomer Herbert Hall Turner, the term “parsec” is a portmanteau of “parallax” and “second,” and is defined as the distance from the Sun to an object that has a one-arcsecond (1⁄3,600 of a degree) parallax. What this awesome-to-say description really means is that if you were to draw a straight line between an object and the Earth, and a straight line between the object and the Sun, if the angle between the lines is one-arcsecond, then the object is one parsec away — or 3.26 light-years. According to Star Wars: The Essential Atlas , the Kessel Run was an 18-parsec (59 light-year) route used by smugglers to get around Imperial blockades.

So why would Solo describe how quickly he traveled it using a word that described distance? It turns out that the expanded universe of the Star Wars franchise — the additional books and content created within the Star Wars universe but outside of the films — contains an answer to that question. The Essential Atlas maps a Kessel Run whose path travels around “The Maw,” a cluster of black holes. To cut down on the distance traveled, pilots could dangerously skirt the edges of the black holes, while trying to avoid spaghettification. If Solo was a skilled enough — or crazy enough — pilot to deviate from the typical route and fly close enough to the black holes to cut nearly 20 light-years off his space odometer, then his ship was fast indeed — the power required to stay out of the gape of an event horizon is something worth bragging about. Image: Star Wars: The Essential Atlas So by being able to dance around singularities, the Millennium Falcon establishes itself as a fast ship — and Solo’s famous brag makes sense. But this brings up a bigger, more inherent problem: The Kessel Run that Solo completed covered nearly 40 light-years of cosmos.

If the blasters and speeders and starships of Star Wars more or less follow the laws of physics, taking that famous run even once would change the entire chronology of Han Solo’s life. One Hour, Three Years In A New Hope, Solo establishes that the Millennium Falcon can go “0.5 past light speed,” dashing the hope for a completely scientifically accurate discussion. Light speed is the universal speed limit, and nothing can outrun it, not even Han’s beloved vessel. Try as you might, E=mc2 is our description of nature’s constant attempt to foil your “jump to hyperspace.” So for the purposes of calculating the Kessel Run, let’s say the Millennium Falcon is the fastest ship ever. Somehow able to withstand the forces involved (perhaps it has something to do with that sweet tractor-beam tech), we can calculate what happens when Han and his baby go 99.9999999 percent the speed of light, or 0.999999999c. t’: This is the amount of time passing for a stationary observer. Because of special relativity, time dilates or expands outward as the moving observer travels faster and faster.

The faster Han goes, the less time he experiences — even if we see him traveling over light years. t: This is the amount of time passing for a moving observer. This is what Han Solo experiences in the Millennuum Falcon. v: This is the velocity of the moving observer — Han Solo. c: This is the speed of light in a vacuum or around 186,000 miles per second. At these ludicrous speeds, time itself contracts. The faster you go, the slower you wade through time’s river. A clock running on a ship moving 99.9999999 percent the speed of light actually ticks more slowly for someone on that ship than a clock for an outside observer. Not only do clocks obey this contraction, but biology does too. Anyone on a hypervelocity ship will age more slowly than those not on the ship. It’s an astonishing conclusion, but it’s how the world actually works. For example, if we transport a super-accurate atomic clock across the globe by plane , we have to correct for the discrepancy between it and another clock on the ground.

After six months in the International Space Station, orbiting astronauts have aged 0.007 seconds less than the rest of us. Unfortunately for Han Solo — and the larger hope of long-distance, high-speed travel — time only contracts for the person who’s moving. It marches on the same for everyone else. Using the equation for time dilation, we can see how much slower Han’s clock ticks while on the Millennium Falcon traveling at 99.9999999 percent the speed of light. Experiencing just one hour on the Falcon, Han returns to find everyone three years older. Because the shortened Kessel Run spans 12 parsecs (39.6 light-years), a ship traveling nearly light-speed would take a little more than 39.6 years to get there. Factoring in time dilation, anyone watching the Kessel Run would see Solo speeding along for almost 40 years, but Solo himself would experience only a little more than half a day.

If you haven’t picked out the potential pitfall for the Star Wars timeline I’ll spell it out: In the time it takes Han to complete just one Kessel Run, the rest of the galaxy battles, negotiates, and force-chokes its way through almost 40 years — and pushes the date of Solo’s birth 40 years further into the past. The Long Con In the Expanded Universe of Star Wars material, the timeline is numbered using the designation BBY, or “Before the Battle of Yavin,” which is the mustering of the rebellion and the destruction of the first Death Star. According to this timeline, the Millennium Falcon was constructed in 60BBY, so we know that there couldn’t have been a Solo Kessel Run in it before then. We also know that Han Solo won the Falcon from Lando Calrissian in a card game at 2BBY. Lastly, we’re told that Han is around 29 years old at 0BBY. A 40-year Kessel Run would mean that Han is chronologically much older than his physical appearance would indicate.

In order to appear 29 years old during A New Hope, Han would have to be 29 years old 40 years before the events of the film. This pushes his birthday back from 29BBY to 69BBY, meaning that Han would have been born before Obi-Wan. Of course, Solo would literally not age a day from when he started the run, but the rest of the galaxy would age without him. Taking the Kessel Run would mean that Solo was entering his teens long before Anakin Skywalker was born, trained, and turned to the Dark Side. In this alternate, physics-friendly timeline, Han Solo and Lando Calrissian are born almost 70 years before they ever lay eyes on Princess Leia’s hair buns. Once they are both around 30, they bet the Millennium Falcon in a card game, which Lando loses (if Han did make a Kessel Run with the Falcon, he would have to have won it long before 2BBY). Han then takes Lando and his prize out for a spin and sets a record by navigating a shorter route close to the black holes of The Maw on a journey to Kessel.

When they return, they remain around 30 years old, but almost four decades have passed, and the Empire is amassing its forces. Of course, there is the warp drive loophole. If you can traverse less distance by folding space itself, there is no time dilation problem. But because Han made that “0.5 past light-speed” remark — and because a warp drive device is never explicitly mentioned in Star Wars, as it is in say, Star Trek: The Next Generation — we still have all the chronological problems that come along with forward-only time travel. And even if Han never personally jumped in the “fastest hunk of junk in the galaxy” and made the Kessel Run in less than 12 parsecs, the chances are still pretty good he made the run without it. The Kessel Run was one of the most trafficked smuggling routes in the galaxy according to additional Star Wars canon. And Han was a seasoned smuggler at the time of A New Hope, already indebted to some unsavory slugs and charmingly shooting his way through Chia Pet-looking aliens. If he didn’t do it with the Falcon, he did it with some other ship. And a slower ship would only move his birth further into the past.

Extending the adventure in pedantry, we can graph the rapidly increasing age of Han Solo as a function of how many Kessel Runs he has been on. If we put his age at 29 in A New Hope, then each Kessel Run would cause his birthday to move further back into history. With only two runs under his belt, Han would be older, chronologically, than Emperor Palpatine. One wonders how a smuggling run with such consequences could even work. Who has the foresight to smuggle something the other party won’t see for 40 years? And imagine how it would work for the smuggler. Off he goes on a Kessel Run, and barely 16 hours later — from his perspective — he returns to find a world changed: the remnants of a galactic clone war, the fall of one Jedi order and the rise of another, and the floating remnants of two Death Stars. < The Cosmic Distance Scale The Cosmic Distance Scale Image Credit: The Isaac Newton Group of Telescopes, La Palma, and Simon Dye (Cardiff University). Because we dwell within the Milky Way Galaxy , it is impossible for us to take a picture of its spiral structure from the outside.

But we do know that our Milky Way has a spiral nature from observations made from within our Galaxy (though whether or not it is a barred spiral is still being debated). To represent this, the beautiful spiral galaxy Messier 74 was used, as it thought to be a similar galaxy to ours. Below is a picture of the real Milky Way taken by the satellite COBE. The disk and center region of our Galaxy are readily recognizable. This image makes the Milky Way appear much more galaxy-like and less like the smudge of stars we see stretching across our night sky. It is possible to imagine what our Milky Way might look like looking down on it from outside. Image Credit: The COBE Project, DIRBE, NASA Distance Information Although the light year is a commonly used unit, astronomers prefer a different unit called the parsec (pc).

A parsec, equal to 3.26 light years, is defined as the distance at which 1 Astronomical Unit subtends an angle of 1 second of arc (1/3600 of a degree) When we use the parsec for really large distances, we often put a prefix in front of it - like kiloparsecs (kpc), which are equal to 1000 parsecs - or Megaparsecs (Mpc), equal to a million parsecs. The Milky Way is about 1,000,000,000,000,000,000 km (about 100,000 light years or about 30 kpc) across. The Sun does not lie near the center of our Galaxy. It lies about 8 kpc from the center on what is known as the Sagittarius arm of the Milky Way. How Do We Calculate Distances of This Magnitude Parallaxes give us distances to stars up to perhaps a few thousand light years. Beyond that distance, parallaxes are so small than they cannot be measured with contemporary instruments.

Astronomers use more indirect methods beyond a few thousand light years. The methods to measure stellar distances greater than a few thousand light years include: Proper motions: All stars move across the sky, but only for nearby stars are these motions perceivable, and even then it takes decades or centuries to measure. Statistically, stars move at about the same rate; therefore, the stars that appear to have larger motions are nearer. By measuring the motions of a large number of stars of a given class, we can estimate their average distance from their average motion. Moving clusters: Clusters of stars, such as the Pleiades and Hyades star clusters , travel together. Analyzing the apparent motion of the cluster can give us the distance to it. Interstellar lines: The space between stars is not empty, but contains a sparse distribution of gas. Sometimes this leaves absorption lines in the spectrum we observe from stars that lie beyond the interstellar gas. (Absorption lines are colors missing in a continuous spectrum because of their absorption by atoms or ions . The spectrum is the array of colors or wavelengths that is obtained when light is dispersed.) The further a star is, the more absorption will be observed, since the light has passed through more of the interstellar medium .

Inverse-square law: The apparent brightness or magnitude of a star depends both on its intrinsic brightness or luminosity (how bright the star actually is rather than how bright it seems) and its distance from us. The inverse-square law says that the flux from a luminous object decreases as the square of its distance. If we know the luminosity of a star (for instance, we have a measured parallax for one star of the same type and know that others of the same type will have similar luminosities), we can measure its apparent brightness and then solve for its distance. There are several variations on this, many of which are used to measure distances to stars in other galaxies. Period-luminosity relation: Some stars are regular pulsators, meaning their intensity changes periodically. The physics of their pulsations is such that the period of one oscillation is related to the luminosity of the star. If we measure the period of such a star, we can calculate its luminosity. From this, and its apparent magnitude, we can calculate its distance. The period-luminosity relation was discovered by Henrietta Swan Leavitt in 1908 when she was studying Cepheid Variable stars in the Magellanic Clouds.

Cepheids, named after Delta Cephei, the first and most luminous of its class to be identified, make excellent distance indicators, because of their periodicity and extraordinary brightness. Not only can they be found at the far reaches of our Galaxy, they can also be resolved in galaxies outside of our own. The most luminous Cepheids can be used to estimate distances to objects as far as 12,000,000 light years away. There are complications in using the period-luminosity relationship. First, the relationship itself depends on the chemical composition of the star. Secondly, the absorption of certain wavelengths of light by the interstellar medium can affect the apparent brightness of the star and therefore must be accounted for. Even with these (and other) complications, Cepheid Variables provide an excellent way to measure the relative distances. To convert to absolute distances, we ideally need to measure the distance to a nearby Cepheid with another, more direct, method. There is much debate at present in this area, in particular regarding the Hipparcos measurements of distances to nearby Cepheids.

(See the Nearest Stars page for more information on Hipparcos measurements.) Interestingly, the size of our own Galaxy was debated for a long while. It was not until early in the 20th century that Harlow Shapley used observations of RR Lyrae variable stars to estimate our Galaxy's size. RR Lyrae stars are similar to Cepheid Variables. They have relatively short periods, typically of about a day or less, and all RR Lyrae stars have approximately the same luminosity. Typically, RR Lyrae stars are less luminous than Cepheids, but they are much more common. Globular clusters of stars - swarms of old stars tightly bound together by gravity and orbiting at the outskirts of galaxies, contain many variable stars, including RR Lyraes. Shapley was able use these to find the distance to the globular clusters that surround our Galaxy. Not only were the globular clusters great distances away, but the Sun did not lie at the center of their distribution, which placed the Sun far from the center of the Galaxy.

Shapley's first estimate of the radius of the Milky Way was off by a factor of 2, but he made an important first step in understanding the nature of our Galaxy. Several more modern methods have been used to map our Galaxy more accurately. The neutral hydrogen gas in our Galaxy emits light at a wavelength of 21 cm; while this light is invisible to our eyes, it is observable to radio telescopes. Other molecules like carbon monoxide also emit radio waves. This is very helpful for mapping the disk portion of our Galaxy. Why Are These Distances Important To Astronomers? Distance is a useful tool on the galactic scale. If you can measure the average speed of stars as they move around the Galactic Center and their distance from the Galactic Center, you can make a plot called a "rotation curve". The rotation curve, which describes the motion of the galaxy can be used to determine the amount of mass within a given radius from the center. The predicted rotation curves for many galaxies (in particular, spiral galaxies like the Milky Way) don't match the observed ones, which led to the discovery of dark matter as an explanation for this discrepancy.

It is thought that these galaxies consist of a large, round halo of dark matter, with the visible matter concentrated in a disk at its center. Travel Time The Voyager spacecraft is traveling away from the Sun at a rate of 17.3 km/s. If Voyager were to travel to the center of our Galaxy, it would take more than 450,000,000 years to travel the 8 kpc. If it could travel at the speed of light , an impossibility due to Special Relativity , it would still take over 26,000 years to arrive! At 17.3 km/s, it would take Voyager over1,700,000,000 years to traverse the entire length of the Milky Way. Even traveling at the speed of light, it would take nearly a hundred thousand years! Scientific Notation & The Metric System The Units of Science Introduction Since Astronomy is one of the oldest sciences, it has developed many traditions over the years. While an astronomer would not hesitate for an instant to apply cutting-edge technology at the telescope or computer, the language and units of measurement in the field change much more slowly.

So, as you set out to investigate the Universe with us, let us introduce you to a few words, units and short-hand that you'll encounter here. Scientific Notation As was discussed in The Scale of the Universe tutorial, the important numbers in Astronomy span almost 40 orders of magnitude in size. Consider the mass of the Sun: MSun = 1,989,000,000,000,000,000,000,000,000,000,000 grams It's cumbersome, to say the least, having to write out all of those zeros. Even kilograms (eliminate 3 zeros) or metric tons (eliminate 6 zeros) don't help much. Furthermore, we really don't know the Sun's mass beyond the accuracy of the fourth digit. All those zeros are just place-keepers, carrying no useful information. For this reason, scientists use a short-hand called Scientific Notation to express very large or very small numbers. In scientific notation the Sun's mass becomes: MSun = 1.989 x 10 33 gm.

The number above the ten, called the power of ten or exponent, stands for the number of decimal places. If it is positive, as in the mass of the Sun, the decimal places are in front ofthe decimal point. So, 1033 means "move the decimal point 33 places to the right and fill the empty places with zeros" (or, more mathematically, multiply by ten 33 times). For very small numbers, such as the mass of the proton, Mp+ = 0.000000000000000000000001673 grams we use negative powers of 10. The mass of the proton becomes Mp+ = 1.673 x 10 -24 grams For negative exponents, the powers of 10 are after the decimal point; 10-24 means "move the decimal point 24 places to the left and fill in with zeros" (or divide by ten 24 times). There are several good web pages about Scientific Notation.

If you would like to read a bit more, try out the University of Maryland's Astronomy Programs site, with a Scientific Notation Exercise and an Astronomical Distance Calculator . Arithmetic in Scientific Notation Arithmetic with Scientific Notation is fairly straightforward, but requires treating the exponent separately. Suppose you wanted to estimate the mass of our Galaxy, the Milky Way. In round numbers there are about a half-trillion stars in the Milky Way: N* N* x Msun 5 x 1011stars . 2 x 1033grams In order to perform the calculation the 2 and 5 are multiplied together and the exponents 11 and 33 are added. MMW ( 2 . 5 ) x 10 (11 + 33) grams 10 x 1044 grams = 1 x 1045grams This is, of course, a crude estimate, especially because we now know that the mass of the Milky Way is dominated by unseen matter. Division in Scientific Notation is just the inverse process. Suppose you wanted to estimate the number hydrogen atoms is the sun.

One estimate would be NH Msun / mH 2 x 1033grams / 1.67 x 10-24grams Now we divide the 2 and the 1.67 (Again this is a crude estimate so we can take mH 2 x 10-24grams) and subtract the exponent of the divisor (-24) from the exponent of the dividend (33). NH If you are very careful you can verify these calculations by longhand. Units of Measurement Before people can share information about the physical world, they need a common language and standard units of measurement. Since science is an international human endevour, scientists all over the world have agreed to use one set of units when they talk with each other about their work. This world-wide standard is called the metric system. So, when Scientist A in Katmandu says the distance to the Sun is 150,000,000 kilometers, Scientist B in Helsinki knows exactly what that means. Here is a list of fundamental units from the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards. The metric system has standard prefixes to indicate relative sizes.

As we noted above, Scientist A reported that the distance to the Sun was 150,000,000 kilometers. The prefix "kilo" means 1,000, so a kilo meter is 1,000 meters. The table below gives some metric names used in these pages and their definitions. Metric Prefixes Here is a comprehensive list of prefixes from the National Institute of Standards and Techonology (NIST). A few examples of how to use the metric prefixes: 10-2 pedes = 1 centipede 10-12 boos = 1 picoboo 10-15 bismols = 1 femtobismol Even the "world standard" metric system has different versions. Astronomers use a version of the cgs (Centimeter-Gram-Second) system, modified for the immense distances in the Universe and huge masses of astronomical objects. Most physicists are converging on use of the MKS (Meter-Kilogram-Second) version or Systeme Internacionale. Length To describe distances and sizes, we define a standard of length. The cgs unit of length is the centimeter, abbreviated as "cm".

A centimeter is between a third and a half an inch: As you can see from the diagram, a centimeter is pretty small - not a very practical unit for the enormous distances in the Universe. If our Astronomer A had to use centimeters to tell Astronomer B the distance to the Sun, it would look like this: 14,959,850,000,000 cm There are three special units of distance used by astronomers. These are the astronomical unit (AU), the light-year and the parsec. The astronomical unit is the average distance of the Earth from the Sun shown above. 1 AU = 1.5 x 1013 cm = 150 million km = 93 million miles = 8.3 "light-minutes" A light-year (ly) sounds like a measure of time, but it is a length - the distance light travels in one year.(We can use a light-year as a unit of measure because ALL light travels at the same speed; it is a fundamental constant of the Universe. More about this later...) So, in one year, light travels: The name parsec comes from the technique of measuring distance called parallax .

The nearest star, Alpha Centauri, is about 1.3 pc or 4 light-years away. We'll mostly stick to light-years in these pages, but you may see stellar distances in pc, distances in the Milky Way Galaxy in kiloparsecs (kpc), distances to other galaxies in megaparsecs (Mpc) , and distances to very distant cosmological objects in gigaparsecs (Gpc). 1 parsec = 3.26 light-years In addition to these distance units, astronomers use the Ångstrom(Å) as a measure of size on the atomic scale. 1 Ångstrom = 10-8 cm. An Ångstrom is about the size of a hydrogen atom. Optical astronomers use the Ångstrom to measure wavelengths of light. The nanometer (nm = 10-9m = 10-7cm = 10Å) is also used as a measure of optical light wavelength, and the micrometer or micron ( µm = 10-6m = 10-4cm = 10,000Å) is used to describe infrared wavelengths.

Mass Everyone knows that the astronauts weigh less when they're walking on the Moon than when they're back on Earth. Since the Moon is less massive than the Earth, it's gravitational attraction is smaller. It is essential to have a unit for measuring "amount of stuff" that would be the same everywhere in the Universe. That unit of "stuff" is called mass. So, an astronaut's weight is less on the Moon, but his/her mass is exactly the same. Actually,weight and mass are two different things. Your weight is the gravitational attraction between you and the Earth. (Or whatever planet you may be visiting.) Your mass is a measure of your inertia , your resistance to changes in motion. Here is a nice explanation of mass. The CGS unit of mass is the gram. It is much smaller than an ounce: 454 grams = 16 ounces = 1 pound The defining standard of mass is a 1-kilogram platinum-iridium cylinder located in the International Bureau of Weights and Measures in Sevres,France. There is a copy of the standard in the National Institute of Standards and Technology in Boulder, and other national laboratories around the world. Should the international standard of the kilogram be redefined?

Read about it here. On the very large (astronomical) scale and the very small (atomic) scale two other units of mass are used. To measure atomic masses the atomic mass unit (amu) is employed. the amu is defined as one-twelfth the mass of a common carbon atom: 1 amu = m(12C)/12 = 1.66 x 10-24g slightly less than the mass of a proton. For stars, galaxies, etc. we use the solar-mass M 2 x 1033g where the " " is the standard astronomical symbol for the sun. The mass of our Milky Way Galaxy is about 1012M . Time The CGS unit of time is the second. The time standard is kept by the NIST using a cesium atomic clock like the one to the right. Learn about atomic clocks. Visit the Atomic Clock tour exhibit. Synchronize your pc clock to the NIST Atomic clock. Atomic clock at NIST Energy Much of astronomy and astrophysics is concerned with understanding the energy generation and luminous energy output from stars, galaxies, etc. The cgs unit of energy is the erg.

An erg is the amount of energy contained in the motion of a 1 gram mass moving with a speed of 1 centimeter/second or about the energy expended by a fly taking off from the wall. The light output of a star or galaxy is called its luminosity measured in ergs/second. The luminosity of our sun L = 4 x 1033 erg/s At the atomic/nuclear level energies are sometimes quoted in electron-volts - eV, the energy of an electron accelerated through a voltage of 1 volt (1 eV = 1.6 x 10-12 erg). The energy-levels and ionization energies of common atoms are a few eV, x-ray photon energies are frequently quoted in kilo-electron-volts - keV, and gamma-rays and nuclear energies are frequently given in MeV. Because of the mass-energy equivalence implied by Einstein's E = mc2 particle masses are sometimes quoted as the equivalent energy, usually in MeV. This is also called the "rest-energy" of the particle. The electron mass, me = 0.511 MeV and the proton mass, mp = 938 MeV.

These are the energies that must be achieved by "atom-smashers" to create these particles out of pure energy.

Question: In what branch of science would a unit of distance called a Parsec be used?
Answer: Astronomy